Packages under /.julia/dev/ shared by different versions of Julia?

Are you actually intending to edit the code in the cloned version of the package? Or are you just trying to have two different versions of A as dependencies for two different versions of Julia?

If the latter, don’t develop A; instead Pkg.add the version that you want to pin for the older Julia version.

Note also that you can always download a new version of A to anywhere you want using any method you want. If you then develop(path/toA) you will use that version of A in that particular environment. develop just means: put an entry into Package.toml that points to a local folder. Whatever code sits there will be used in that environment. That could be your own code or a package that you downloaded (by whatever method) from anywhere else.